I made my Buttercream tonight and it calls for 3T of flavoring. I usually use 2 of vanilla and 1 of butter and it's awesome. However, I ran out of the Wilton butter and picked up the Watkins butter last night because I had read good things about it. I used 2T of vanilla and 1 of butter and good grief it's awful! I'm completely disgusted, because this recipe made 12 cups of frosting and I only iced 1 8" cake! I do not want to throw all of this away, but I'll never use it again unless I can somehow fix this.
Take a cupful and add a LOT of bittersweet chocolate, like 3 ounces. If that works OK then do it to the rest. If not, there is nothing to rescue the rest of this batch.
But take the bottle back to where you bought it if possible. Some extracts go bad even in sealed bottles. I threw away a couple ounce bottles of LorAnn flavour once--it had gone all bitter and fortunately I tasted it before I added it to the cake.
I use Watkins all the time and have been really pleased with it. I use 2 tsp of butter flavor and one tsp of almond. Sorry this happened to you. I guess just play with it and see what happens. Good luck!